The takeoff procedure, from releasing the brakes until the aircraft actually takes off, constitutes a delicate aircraft operating stage during which, owing to losses of performance and changes as regards the direction and strength of the wind or for other reasons, accidents or incidents, whose probability has been assessed at about 10.sup.-6 per take-off, may possibly occur. Accordingly, in order to improve safety, it would be essential to provide the aircrew at the right time with accurate specific information enabling the aircrew to decide on whether or not to interrupt or correct the take-off procedure before the aircraft travelling on the runway reaches a critical speed obtained by a theoretical calculation, said speed being defined as the speed at which take-off may be interrupted and beyond which take-off must be continued.
So as to more readily understand this notion of critical speed denoted subsequently by V.sub.1, it would be first of all proper to define various distances characteristic of an airport runway. In fact, the runway and its immediate surroundings can be divided into several zones depending on their function:
the length of the available runway, which is the runway length able to accomodate the aircraft in all use conditions,
the stopping zone, which is an extension of the actual runway able to be used as an aerodrome taxi circuit on the ground, but also allowing for breaking should the take-off procedure be interrupted,
the clear zone, which is the zone in the immediate extended part of the runway, said extended part being able to be taken into account in calculating the takeoff distance.
Account is then taken of the various distances relating to the take-off procedure for a given aircraft:
the running distance, which is the distance, in relation to the ground, covered by the aircraft between releasing the brakes and that half of the "segment" defined by the point reached by the aircraft at the speed when it leaves the ground and the point corresponding to the rise of the aircraft to an altitude defined in the certification regulations, namely 35 feet (10.7 meters). It needs to be less than or equal to the available runway length;
the take-off distance, which is the distance, in relation to the ground, covered by the aircraft between releasing the brakes and the rise of the aircraft to said altitude of 35 feet. Should an engine breakdown occur at V.sub.1 +.epsilon. (.epsilon. being the mathematical sign denoting an immeasurably small value), it needs to be less than or equal to the sum:
available runway length+clear zone;
the acceleration/stopping distance, which is the distance covered by the aircraft between releasing the brakes and the stoppage of the aircraft by using its own brakes should an engine breakdown occur at V.sub.1 -.epsilon..
This distance needs to be at the most equal to the sum: EQU available runway length+stoppage zone.
From the moment the aircraft reaches the critical speed V.sub.1 defined by calculation before take-off and set for take-off, the take-off procedure must strictly be continued. In fact, in the opposite case, the stoppage of the aircraft could not be obtained before the end of the stoppage zone, as defined above. Up until now, the pilot compares the speed of the aircraft with said critical speed. The critical speed V.sub.1 is a speed defined with respect to the air, which results in a first risk when the wind suddenly varies during take-off. However, independently of any wind change, although it is relatively simple for the pilot to verify the notion of the speed V.sub.1, in actual fact this notion is linked to a notion of distance, currently unknown to the aircrew, which is clearly a critical parameter as regards take-off. Consequently, any deterioration of the performances of the aircraft with respect to theoretical performances implies that the aircraft shall reach said critical speed after having covered a distance longer than the calculated theoretical distance corresponding to this critical speed. In truth, as things currently stand, the pilot who decides to interrupt the take-off procedure at V.sub.1 -.epsilon. may not be certain that he could stop the aircraft without risking leaving the runway.